This invention relates to nondestructive testing and in particular to a method and apparatus for inspecting laminar structures for defects in bonding between layers.
Coal is pulverized in a mill by passing the coal between a rotating bowl and a plurality of pulverizer rolls. The pulverizer rolls experience erosion as the coal is crushed and thus require a surface that is fabricated of wear resistant material. Rather than fabricate the entire roll of wear resistant material, the body is fabricated of gray iron and one or more layers of a clad material, chosen for its particular wear resistant property, are applied over the base material of gray iron. The clad construction is more economical than fabricating the entire roll from the wear resistant material. The clad material is often applied to the base material in the form of a hard facing or layer of weld material applied to the bodies outer surface. This method of construction has application both in the manufacture of new pulverizer rolls as disclosed in U.S. Pat. No. 4,389,767 and in the remanufacture of pulverizer rolls that have eroded in operation.
A problem has been encountered in using the laminar construction rolls. Due to defective bonding between layers of clad material or between a layer of clad material and the body of a pulverizer roll, the clad material in the region of the defective bond separates from the roll surface leaving a corresponding recessed area which reduces the efficiency of the pulverizer.
Therefore, a need exists for a simple, reliable nondestructive method and apparatus for detecting the presence and location of disbond flaws between clad layers or between the clad layer and the body of the pulverizer roll. Such a nondestructive method and apparatus would be useful to inspect pulverizer rolls of laminar construction as part of a manufacturing process or as part of a field inspection of equipment during routine maintenance.